This stops the Salesforce CTI adapter from loading in the browser.
As the Firefox blocker cannot be configured to always allow content from a particular site, Salesforce advise it is currently necessary to disable the blocker.
In the URL bar enter:
Accept the warning to be careful if it appears
Then filter the list by entering:
Into the search bar.
click on the result and select Toggle to turn the Mixed Content block
Salesforce are raising this with Firefox.